In many applications where electric motors are used, a smooth rotation of the rotor of the electric motor, and therefore also a precise control of the rotor current and the wave shape thereof, are required. To be able to achieve a sufficiently precise control, it is required that the angular position of the rotor can be detected in an accurate manner. Usually, in conventional electric motors, an angle sensor or resolver, together with associated electronics, are used for determining the angular position. The angle sensors detect mechanical movement and convert the detected movement into electric signals. Optical pulse sensors and absolute angle sensors are two commonly occurring types of angle sensors.
To be able to achieve a precise current control and smooth rotation in so-called brushless DC motors, i.e. alternating current motors with permanent magnets on the rotor and electronic commutation, accurate detectors are required for the detection of the angular position of the rotor and the electronic commutation. For example, US2010/0090633 A1 discloses a motor assembly comprising a brushless DC motor having a rotating shaft, an electronic controller attached to the motor and positioned outside the rotating shaft, and a two pole permanent magnet affixed to the shaft for rotation by the shaft in a plane orthogonal to the axis or rotation of the shaft. Furthermore, the motor assembly comprises a X-Y Hall Effect Sensor carried by the electronic controller and positioned proximate the magnet, wherein the Hall Effect Sensor produces Sine and Cosine components of the magnetic field as the magnet is rotated by the motor shaft. The electronic controller further includes means for determining the motor angle position from said Sine and Cosine components, and commutation logic. A controller on the board, positioned directly above one end of the rotating shaft of the motor assembly, contains the highly integrated functions: internal analog digital converters, pulse width modulation registers for driving the power amplifier, internal communication ports, and all of the RAM memory and non-volatile Flash memory that is required for motor control.
EP 1 099 092 B1 discloses a brushless electric motor and a method for controlling it. The electric motor comprises a rotor, a sense element with a plurality of magnetic poles, and first and second Hall effect sensors mounted with their sensing planes perpendicular to a surface of the sense element to measure magnetic flux from the magnetic poles in a direction tangential to the sense element, wherein one of the sense element and the first and second Hall effect sensors is mounted in a fixed relationship with the rotor. The Hall effect sensors are adapted for outputting a measurement signal so that at least one output signal from at least one of the first and second sensors is linear at each rotational position of the rotor, wherein the measurement signals are used for determining an absolute rotational position of the rotor within an electrical cycle, wherein the motor is adapted to be controlled based on the decoded rotor position.
U.S. Pat. No. 7,579,799 B2 discloses a system for measuring the angular position of a rotor, a method for measuring the angular position, and a method for controlling an electric motor. The system has a rotor assembly comprising a plurality of annularly disposed magnetic domains of the magnetic poles of the motor, and a sensor module having at least two Hall Effect Devices disposed at a specified relative angular separation with respect to each other. The sensor module is suitably configured to produce a plurality of input signals in response to a magnetic flux variation corresponding to axial rotation of the magnetic domains of the magnetic field poles of the motor. A position measurement module is suitably configured to process said input signals to produce converted sinusoidal reference signals. The sinusoidal reference signals have at least one of an offset scale factor and an amplitude scale factor. An error term is determined by processing said input signals to produce a position estimate signal corresponding to an approximate measurement of the angular position of the rotor. Subsequently, a refined measurement of the angular position of the rotor is produced by processing the error term.
Furthermore, U.S. Pat. No. 6,906,494 B2 discloses a motor controller for driving a motor having a rotor, which includes magnets. The motor controller comprises a magnetic flux detector for detecting a magnetic flux from the magnets and obtaining a magnetic pole signal, a position signal converter for finding a position of the rotor based on the magnetic pole signal, a differentiator for finding a speed signal based on the output of the position signal converter, a speed controller for comparing the speed signal with a predetermined instructive speed, and for outputting an instruction signal, and a pulse width modulation controller for performing pulse width modulated drive of the motor according to the instruction signal. The pulse width modulation controller drives the motor in a sine wave form, wherein the magnetic pole signal is a 2 phase sine wave signal having a phase difference of 90°.
A disadvantage of many of the previously known motor assemblies with brushless DC motors and integrated control electronics is that the motor assemblies are often larger and bulkier than desirable for many applications. One reason for this is that a conventional brushless DC motor generally produces a rather low torque, which means that a larger motor size will have to be chosen, or that the motor will have to be equipped with a gearbox, to obtain a motor with a sufficiently large output torque. Another reason is that many of the previously known motor assemblies have a design which requires a complicated and space-consuming measurement arrangement, usually disposed at the shaft end of the motor, to be able to measure the angular position of the rotor and/or bulky control electronics, which considerably increases the length or other external dimensions of the motor assembly.